Arrangement for comparing electrical characteristics



Dec. 5, 1950 c. B. SHEPPARD 2,532,736

ARRANGEMENT FOR COMPARING ELECTRICAL CHARACTERISTICS Filed Aug. 21, 1946WAVE-SIGNAL GENERATOR INVENTOR,

.CHARLES B. SHEPPARD,

Patented 5, 1950 PATENT OFFICE ARRANGEMENT FOR COMPARING ELECTRICALCHARACTERISTICS can-lee B. Sheppard, Cheltenham, Pa.. asaignor toHazeltine Research, Inc., Chicago, 11]., a corporation of IllinoisApplication August 21, 1946, Serial No. 692,073

-8 Claims. Y 1

The present invention relates to arrangements for comparing, at highfrequencies, electrical characteristics such as the electricalimpedances of two elements.

Heretofore standing-wave measuring apparatus 5 of the slotted-line typehas been utilized to determine, at high frequencies, the magnitude andthe phase of the impedance of an electrical element. An arrangement ofthis type is described in detail and claimed in a copending ap- 1oplication filed in the name of David Dettinger, Serial No. 573,436,filed January 18, 1945, entitled "Standing-Wave Measuring Apparatus,"now U. S. Patent No. 2,454,042, issued November 16, 1948, and assignedto the same assignee as. the 13 instant invention. Such arrangementsgenerally comprise a coaxial transmission line having a slotted outerconductor and a centrally disposed inner conductor which interconnect ahigh frequency signal generator and the impedance to be measured. Anadjustable carriage is slidably mounted on the outer conductor andcarries a vertically adjustable pickup device which is connected to avacuum-tube, voltmeter. The adjustable carriage co-operates with a scaleor scales on the outer conductor in a manner to indicate the positionand the amount of movement ofthe pickup device, while the vaccum-tubevoltmeter indicates the magnitude of the voltages coupled to the pickupdevice at various positions of the carriage along the slotted outerconductor. To determine the characteristic of an unknown impedance,observations of several scale and meter readings, computations makinguse of this observed data, and the utilization of impedance conversioncharts are ordinarily required. Absolute values of impedances are thenobtained by this rather laborious procedure. While such arrangements aresatisfactory for use in the laboratory by skilled operating personnel,they are generally too diflicult to operate and too slow for use in thefactory by relatively unskilled individuals for comparing the impedancesof mass production items with that of a known standard, particularlywhen comparisons over a relatively wide range of frequencies aredesired.

It is an object of the present invention, therefore, to provide a newand improved arrangement for comparing, at high frequencies, anelectrical characteristic of two elements which arrangement avoids oneor more of the disadvantages and limitations of prior arrangements ofthe type described.

It is another object of the invention to provide a new and improvedarrangement for compar g, at higlf frequencies, .the electricalcharacteristics of .two impedances, which arrangement may readily bevoperated by relatively unskilled personnel in a manner to obtainaccurate and reliable comparison information.

It is a further object of the invention to provide an arrangement foraccurately checking, over a, relatively wide range of high frequencies,the variation in the impedance of an electrical element from that of aknown standard element and at a rate which is faster than heretofore hasbeen possible.

In accordance with a particular form of the invention, an arrangementfor comparing at high frequencies electrical characteristics of twoelements comprises a wave guide and coupling means by which each of theelements may be coupled to the wave guide at individual ends thereof.The arrangement also includes means for supplying high frequency wavesignals to the wave guide, at a point equidistant the ends, to supplyhigh frequency wave signals to the elements. The arrangementadditionally includes relation to the equidistant point for derivingwave signals having electrical characteristics dependent on theelectrical characteristics of the two elements. The arrangement furtherincludes means responsive jointly to the last-mentioned wave signals forobtaining a comparison of the electrical characteristics of theelements.

For a better understanding of the present invention, together with otherand further objects thereof, reference is had to the followingdescription taken in-connection-with the accompanying drawing, and itsscope will be pointed out in the appended claims.

Referring now to the drawing, there is illus-= trated schematically anarrangement embodying the present invention for comparing electricalcharacteristics. This has particular utility for comparing at highfrequencies, the electrical characteristics of two elements It and I l,specifically the impedances thereof, and comprises a wave guide 52 ofthe coaxial transmission-line type. Element It preferably isan-fimpedance having a known magnitude and phase characteristic, hence,it will be referred to hereinafter as the standard impedance. Element itmay comprise a wave-signal antenna, the impedance of which is unknownand is to be compared with, or adjusted to, that of the standardimpedance it. Wave guide 52 includes a main portion comprising an outerconductor is and a centrally disposed. inner conductor it. Thearrangement also includes coupling means by which each of the impedancesit and it may be coupled to the wwt gu de it at spaced points thereon.This pose to be explained subsequently.

means comprises two pairs of terminals IS, IS and I 5, I5 disposed atopposite ends of the wave guide I2. Preferably the wave guide I2 has asubstantially uniform characteristic impedance between its opposite orterminal ends for a pur The impedance comparing arrangement alsoincludes means for supplying high frequency wave signal to the waveguide I2, at a point 35 between or intermediate the above-mentionedterminals I5, I5 and I6, I6, in order to supply high frequency wavesignals to the impedances I and II. This'means comprises a highfrequency wave-signal generator I8, which is preferably one that may beadjusted to operate over a relatively wide range of frequencies.Wave-signal generator I8 is connected by an auxiliary portion I9 of thewave guide I2 to the above-mentioned intermediate point 35. Since thetwo sec-.

. tionsof the main portion of the wave guide I2,

each of which may be considered to have a characteristic impedance equalto Z, are efiectively connected in parallel with each other across theauxiliary portion I9, the latter preferably has a characteristicimpedance of Z/2 to afiord a proper impedance match at the junction ofthe two portions. Likewise it'is preferable that the generator I8 have acharacteristic impedance equal Z/Z.

The arrangement of the instant invention also includes directionalcoupling means coupled to the wave guide I2 betweeneach of the spacedpoints comprising the first pair of terminals I5, I and the second pairof terminals IE, IS and the intermediate point 35. This directionalcoupling means is so constructed that it is responsive to high frequencywave signals traveling along the wave guide I2 only in a predetermineddirection with relation to the intermediate point 35 for derivingwave-signal energy having electrical characteristics dependent on theelectrical characteristics of the two impedances I0 and II. One of thesemeans comprises a unidirectional coupling loop which is preferablypositioned between the junction 35 and one end of the main portion I3,while the other means comprises a similar coupling loop 2| which islocated at a corresponding position between the junction 35 and theother end of the main portion I3. Hence, the coupling loops 20 and 2|are mounted in symmetrically disposed positions along the wave guide I2.One end of the coupling loop 20 is connected through a terminatingresistor 22 to the inner surface of the outer conductor I3 while thecorresponding end of coupling loop 2| is similarly connected to theouter conductor I3 through a terminating resistor 23. Each of thecoupling loops 20.and 2| is efi'ectively responsive only to highfrequency wave signals translated by the wave guide I2 in apredetermined direction, to be referred to more fully hereinafter. Thecoupling loops 20 and 2I may be of the type described in detail andclaimed in the copending application of Harold A. Wheeler, entitledWave-Signal Translating System, Serial No. 670,081, filed May 16, 1946,and assigned to the same assignee as the instant invention. The couplingloops 20 and 2| include output terminals 25 and 2B, respectively, forapplying wave signals derived by the coupling loops to an outputcircuit.

The impedance comparing arrangement also includes means responsivejointly to the two wave signals derived by the coupling loops 20 and 2|to obtain a comparison of the electrical characteristics oftheimpedances III and II. This means comprises a wave-signal rectifier,for example a crystal detector, one terminal of which is connectedto theoutput terminal 25 while the other terminal thereof'is connected to theother output terminal 26 through a condenser 21. The direct current pathfor unidirectional potentials derived by wave-signal rectifier 28includes the series combination of a meter 29 of conventionalconstruction, and a pair of radio-frequency chokes 30 and 3 I the inputterminal of each of the latter elements being connected to a differentterminal on the wave-signal rectifier 28. The impedances of the couplingloops 20 and 2| and the input terminals of the wave-signal rectifiersystem are preferably matched to those of the output terminals 25 and 26so as to avoid undesired wavesignal reflections from any of theseterminals. The impedance comparator of the instant invention aflordsextremely reliable information when the coupling loops 20 and 2|, theresponsive means including the wave-signal rectifier 2B and meter 29,and the impedances I0 and II under comparison are symmertically disposedwith respect to the junction point 35 of the inner conductors of theauxiliary portion I9 and the main portion I3 of the wave guide I2.

Considering now the operation of the abovedescribed impedance-measuringsystem, it will be initially assumed that the impedance mismatches ofimpedances III and I I with respect to the transmission line I2 areequal and of the same phase. In accordance with practical designconsiderations, the impedances of the elements which are being compared,usually do not match that of the wave guide I2 to which they areconnected. Since the wave-signal generator I8 is capable of beingoperated over a relatively wide range of frequencies, it is set at apredetermined frequency and high frequency wave signals are translatedto the main portion of wave guide I2 by means of the auxiliary portionI9. These wave signals are translated in difierent directions from thejunction point 35 and are applied to the imped-' ances I 0 and II bymeans of the terminals I5, I5 and I6, I6. It is a characteristic of eachof the coupling loops 20 and 2| that wave-signal energy, which iscoupled thereinto by wave signals traveling in a direction toward theimpedances I0 and II, is dissipated in eachof the terminating resistors22 and 23. Consequently, output signals are not applied to the outputterminals 25 and 26 in response to wave signals which are beingtranslated by the wave guide I 2 to the terminals I5, I5 and I6, I6.Since the impedance values of the impedances I0 and II are mismatchedwith respect to the main portion of the wave guide I2, wave-signalreflections therefrom are produced at the terminals I5, I5 and I6, I5and are translated by the wave guide I2 in a direction toward thejunction point 35 of the main and auxiliary portions of the wave guide.Waifiasignal energy is derived by each of the unidirectional couplingloops 20 and 2| from these reflected wave signals and is applied by theconnections to the terminals 25 and 2G to the two terminals of therectifier 28. Since the electrical lengths of the wave signal pathsthrough corresponding portions of the wave guide I2 and the rectifier 28of the indicating system are equal, and also because the impedancemismatch of the standard impedance I0 and the unknown impedance II withrespect to the wave guide I2 are equal, the high frequency wave signalsapplied to the rectifier 28 are of equal magnitude and phase. Thus aunidirectional output is not proawaken duced by the rectifier 20 forapplication to. the meter 29. Consequently, the absence of a reading onthe meter 29 indicates that the impedances m and II are equal in phaseand magnitude at this particular frequency. This operation may berepeated for each of several frequencies comprising a range offrequencies by making suitable adjustments of the wave-signal generatorl8 and observing the indication on the meter 29. The absence of anyreading on the meter 28 will indicate that the unknown impedance llequals that of the known impedance In over a range of frequencies.

Due to mass production methods, however, some deviation of the impedanceof the unknown impedance, for example an antenna, from the known orstandard impedance is ordinarily experienced. Assuming now that theunknown impedance II is not equal to that of the standard impedance land that both impedances are mismatched with respect to the wave guidel2, the application of wave signals from the generator 68 to theterminals l5, l and l6, it will produce reflected wave signals from theimpedances Ill and ii of unequal magnitude or phase, or both.Unidirectional coupling loops and 2! will transfer to the outputterminals 25, 26 wave signals having different characteristics inresponse to the reflected wave signals, and the transferred signals willbe applied to the rectifier 28. The net high frequency wave signalapplied to rectifier 28 will be the vector difference between the wavesignals at output terminals 25, 26, and the unidirectional output signalderived by the rectifier will be applied to the meter 29 through theradio-frequency chokes 30 and 3!. Meter 29, therefore, indicates thevector difference between the signals applied to the rectifier 28 and,hence, the impedance deviation of the unknown impedance it with respectto the standard impedance it.

It will be manifest that wave-signal generator 08- may be adjusted in awell-known manner to operate at a number of different frequencies anddeterminations made of the impedance variations of the unknown impedanceH with respect to the standard impedance ill at each of thosefrequencies. Since the high frequency wave-signal paths to the rectifier28 are symmetrical and equal in length, the arrangement is substantiallynonfrequency selective over a, relatively wide range of highfrequencies. It will also be apparent that meter 29- may be calibratedin terms of impedance variations, so that an unknown impedance l I maybe discarded as unsatisfactory or corrected by suitable adjustmentthereof when its impedance value does not fall within certain prescribedlimits.

From the foregoing explanation of the operation of the invention, itwill be understood that the wave signals to be applied to thewave-signal rectifier desirably comprise only those signals which arereflected from the impedances l0 and M which are being compared.Accordingly, it is preferable that the main body portion of the waveguide i2 have a uniform impedance throughout its length to avoid anyundesired re- Elections from discontinuities therein.

It will be apparent from the foregoing description that the impedancecomparator of the instant invention is particularly adapted for use inmaking quick and accurate checks on the variations of an unknownimpedance from a, standard impedance over a relatively wide range offrequencies. 4

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from'the invention, and it is, therefore,aimed in the appended claims to cover all such changes and modificationsas fall within the true spirit and scope of the invention.

What is cla-med is:

1. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprising: a wave guide; coupling meansby which said elements may be coupled to said wave guide at spacedpoints thereon; means for supplying high frequency wave signals to saidwave guide, at a point equidistant said spaced points, to supply highfrequency wave signals to said elements; individual directional couplingmeans coupled to said wave guide in corresponding relationships betweenindividual ones of said spaced points and said equidistant point andresponslve to reflected high frequency wave signals traveling along saidwave guide between individual ones of said spaced points and saidequidistant point but only in a direction away from said first-mentionedcoupling means for deriving wave signals having electricalcharacteristics dependent on said electrical characteristics of said twoelements; and means responsive jointly to said last-mentioned wavesignals for obtaining a comparison of said electrical characteristics ofsaid elements.

2; An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprsing: a wave guide; coupling meansby which said elements may be coupled to said wave guide at spacedpoints thereon; said wave guide having a substantially uniformcharacteristic impedance between said spaced points to min-.

imize undesirable high frequency wave-signal refiections between saidspaced points; means for supplying high frequency wave signals to saidwave guide, at a point equidistant said'spaced points. to supply highfrequency wave signals to said elements; individual drectional couplingmeans coupled to said wave guide in corresponding relationships betweensaid spaced points and said equidistant pont' and responsive to highfrequency wave signals traveling along said wave guide betweenindividual ones of said spaced points and said equidistant pont but onlyin a predetermined direction with relation to said equidistant point forderiving wave signals having electrical characteristics dependent onsaid electrical characteristics of said two elements; and meansresponsive jontlyto said last-mentioned wave signals for obtaining acomparison of said electrical characteristics of said elements.

3. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprising: a wave guide; coupling meansby which said elements may be coupled to said wave guide at individualends thereof; means for supplying high frequency wave signals to saidwave guide, at a point equidistant said ends, to supply high frequencywave signals to said elements; individual directional coupling meanscoupled to said wave guide in corresponding relationships betweenindividual ones of said ends and said equidistant point and responsve tohigh frequency wave signals traveling along said Wave guide betweenindividual ones of said ends and said equidistant point but only in apredetermined direction with relation to said of said electricalcharacteristics of said elements.

4. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprising: a wave guide; coupling meansby which said elements may be coupled to said wave guide at spacedpoints thereon; means for supplying high frequency wave signals to saidwave guide, at a point intermediate said spaced points, to supply highfrequency wave signals to said elements; individual directional couplingmeans coupled to said wave guide between individual ones of said spacedpoints and said intermediate point and responsive to high frequency wavesignals traveling along said wave guide between individual ones of saidspaced points and said intermediate point only in a predetermineddirection with relation to said intermediate pofnt for deriving wavesignals having electrical characteristics dependent on said electricalcharacteristics of said two elements; means responsive jointly to saidlast-mentioned wave signals for obtaining a comparison of saidelectrical characteristics of said elements; said spaced points, saidindividual directional coupling means, and said responsive means beingsymmetrically disposed electrically with respect to said intermediatepoint.

5. An arrangement for comparing. over a relatively wide range of highfrequencies, electrical impedances of two elements comprising: a waveguide; coupling means by which said elements may be coupled to said waveguide at spaced points thereon; means for supplying high frequency wavesignals to said wave guide, at a point intermediate said spaced points,to supply high frequency wave signals to said elements; individualdirectional coupling means coupled to said wave guide in correspondingrelationships between individual ones of said spaced points and saidintermediate point and responsive to high frequency wave signalstraveling along said wave guide between individual ones of said spacedpoints and said intermediate point only in a predetermined directionwith relation to said intermediate point for deriving wave signalshaving electrical characteristics dependent on said electricalcharacteristics of said two elements; and substantially nonfrequencyselective means responsive jo'ntly to said last-mentioned wave signalsfor indicating the vector difierence between said impedances.

6. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprising: a wave guide including amain portion having two ends and having a characteristic impedance equalto Z and including an auxiliary portion having a characteristicimpedance equal to Z/2 symmetrically disposed intermediate said two endsof said main portion; coupling means by which said individual ones ofsaid elements may be coupled to said individual ones of said two ends ofsaid main portion; means for supplying high frequency wave signals tosaid auxiliary portion to supply high frequency wave sgnals to saidelements; unidirectional coupling means individually coupled to saidwave guide at each of two positions symmetrically dis.-

tel

posed between said auxiliary portion and each of said ends of said mainportion and responsive to high frequency wave signals traveling alongsaid wave guide between individual ones of said ends and said auxiliaryportion but only in a predetermined direction with relation to saidauxiliary portion for deriving wave signals having electricalcharacteristics dependent on said electrical characteristics of said twoelements; and means responsive jo'ntly to said last-mentioned wavesignals for obtaining a comparison of said electrical characteristics ofsaid elements.

7. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprisng: a wave-guide system havingtwo guide portions; coupling means by which individual ones of saidelements may be coupled to individual ones of said wave-guide portions;means equidistant said coupling means for supplying high frequencywave-signal energy through individual ones of said wave-guide portionsto individual ones of said elements; individual directional couplingmeans coupled to each of said wave-guide portions in correspondingrelationships and responsive to high frequency wave-signal energytraveling therethrough between individual ones of said coupling meansand said supply means but only in a predetermined direction w;threlation to said supply means for deriving wave-signals havingelectrical characteristics dependent on said electrical characteristicsof said two elements; and means responsive jointly to said derivedlast-mentioned wave s gnals for obtaining a comparison of saidelectrical characteristics of said elements.

8. An arrangement for comparing, at high frequencies, electricalcharacteristics of two elements comprising: a wave-guide system havingtwo guide portions; coupling means by which individual ones of saidelements may be coupled to individual ones of said wave-guide portions;means equidistant said coupling means for supplying high frequencywave-signal energy through individual ones of said wave-guide portionsto individual ones of said elements; a pair of directional couplingmeans individually coupled to individual ones of said wave-guideportions in corresponding relationships and responsive to high frequencywave-signal energy traveling therethrough between individual ones ofsaid coupling means and said supply means but only in a predetermineddirection with relation to said supply means for deriving wave signalshaving electrical characteristics dependent on said electricalcharacteristics of said two elements; and means responsive jointly tosaid derived lastmentioned wave signals for obtaining a comparison ofsaid electrical characteristics of said elements.

CHARLES B. SHEPPARD.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,422,601 Tashjian June 17, 19472,423,416 Sontheimer et al. July 1, 1947 2,425,084 Cork et a1. Aug. 5,1947

